Geophysical logging tools, particularly ultrasonic acoustic borehole image techniques, are applied on two different wells (CP1 and CP2) to identify and characterize the lacustrine interval (40m) of the Camp dels Ninots maardiatreme infill (Pliocene, Catalan Coastal Ranges). The CP1 well was cored and also geophysical logs and oriented borehole images were acquired. CP2 hole was partially cored but logging (geophysics and borehole images) was fully undertaken. Continuous core recovery in CP1 is compared with oriented images and is further used to identify and characterize highly reflective signals in a section of CP2 borehole that was not cored. These signals are related to silicified zones and belong to discontinuous centimetric chert nodules, while indurated massive carbonates are characterized as intervals of continuous reflectance. Despite opal nodules (chert) can be relatively small, they have a
distinctive response in the ultrasonic borehole images.Peer Reviewe

Agustí, Jordi

Anadón, Pere

Campeny, Gerard

Crespo, José

Gómez de Soler, Bruno

Ibáñez Insa, Jordi

Jurado, Maria José

Martín-Martín, J. D.

Miró, Jordi

Oms, O.

Digital.CSIC

IX Congreso Geológico de España                                                                                                                   Geo-Temas, 16 (2) ISSN 1576-5172 575 Borehole image techniques applied to identification of chert and dolomite layers in lacustrine sediments  Técnicas de imagen de sondeo aplicadas a la identificación de intervalos chert y dolomita en sedimentos lacustres   J. Miró1, M.J. Jurado2, O. Oms3, J.D. Martín-Martín1, J. Ibáñez2, P. Anadón2, J. Crespo2, B. Gómez de Soler4, G. Campeny4 and J. Agustí5  1 Barcelona University, Faculty of Geology. Martí i Franquès, s/n. 08028 Barcelona (Spain), jmiropad@gmail.com,juandiegomartin@ub.edu 2 Institute of Earth Sciences Jaume Almera, CSIC, 08028 Barcelona (Spain), mjjurado@ictja.csic.es, jcrespo@ictja.csic.es, jibanes@ictja.csic.es, panadon@ictja.csic.es 3 Autonomous University of Barcelona.  Science Faculty, Geology Department. Campus Bellaterra 08193 (Spain). joseporiol.oms@uab.cat  4 Institute of Human Paleocology and Social Evolution, Campus Sescelades URV, 40007 Tarragona (Spain), bgomez@iphes.cat, gcampeny@iphes.cat 5 ICREA. Institute of Human Paleocology and Social Evolution, Campus de Sescelades, 40007 Tarragona (Spain), jordi.agusti@icrea.cat  Abstract: Geophysical logging tools, particularly ultrasonic acoustic borehole image techniques, are applied on two different wells (CP1 and CP2) to identify and characterize the lacustrine interval (40m) of the Camp dels Ninots maar-diatreme infill (Pliocene, Catalan Coastal Ranges). The CP1 well was cored and also geophysical logs and oriented borehole images were acquired. CP2 hole was partially cored but logging (geophysics and borehole images) was fully undertaken. Continuous core recovery in CP1 is compared with oriented images and is further used to identify and characterize highly reflective signals in a section of CP2 borehole that was not cored. These signals are related to silicified zones and belong to discontinuous centimetric chert nodules, while indurated massive carbonates are characterized as intervals of continuous reflectance. Despite opal nodules (chert) can be relatively small, they have a distinctive response in the ultrasonic borehole images.  Key words: Borehole imaging, acoustic televiewer, silicification.   Resumen: En este estudio se han aplicado técnicas de testificación geofísica de sondeos y, en especial el televiewer acústico, en dos sondeos (CP1 y CP2) para identificar y caracterizar el intervalo lacustre (40m) del relleno del maar-diatrema del Camp del Ninots (Plioceno, Cadenas Costeras Catalanas). El sondeo CP1 se ha testificado con sondas geofísicas y se ha perforado con recuperación continua de testigo (wireline). El sondeo CP2  solo se recuperó parcialmente pero se ha testificado en su totalidad (geofísica y sondas de imagen orientada). La recuperación continua de CP1 se ha comparado con las imágenes de testificación y se han utilizado para identificar en el pozo CP2 niveles reflectivos del tramo donde no se recuperaron testigos. Los niveles reflectivos están relacionados con zonas silicificadas y se corresponden a nódulos discontinuos centimétricos de chert y los tramos de carbonato masivo se reconocen  como nieles reflectivos continuos. A pesar de que los nódulos de ópalo (chert) pueden ser relativamente pequeños, su reflectividad facilita su identificación sobre las imágenes del televiewer acústico.  Palabras clave: sondas de imagen orientada, televiewer, silicificación.   INTRODUCTION  Fresh water lake environments affected by volcanism are commonly characterized by high Mg and Si activity coupled with high carbonate alkalinity and microbial influence, which can potentially generate a complex set of abiotic and biotic interactions and mineral products (Wright, 2012). Some of these early diagenetic mineral products include opal nodules and dolomite.  Here we explore borehole imaging techniques and sonic logging as a potential tool to identify these silicified and dolomite horizons without continuous rock recovery. Therefore, in this study we compare logging coupled with core recovery to characterize and better interpret further wells where logging is the only available data. This is applied to the Camp dels Ninots maar.  GEOLOGICAL SETTING  The Camp dels Ninots maar-diatreme (CNMD) is a Pliocene volcano located near Caldes de Malavella (Girona, NE Spain) and is located at the margin of the La Selva Basin (Pous et al. 1990). This volcano is part of the Catalan Volcanic Zone (Gómez de Soler et al., 2012), one of the European Rift System provinces (Figure 1). The CNMD origin is related with the NW- IX Congreso Geológico de España                                                                                                                   Geo-Temas, 16 (2) ISSN 1576-5172 576   FIGURE 1: A) Location of the Camp dels Ninots maar diatreme within the Iberian Peninsula (upper left) and within the Catalan Volcanic Field. B) Simply geological map of Camp dels Ninots maar. C) Topographic map of the Camp dels Ninots maar showing the location of the CP1 and CP2 wells.  A &B by Oms et al., 2015 (modified)   SE fault system, which limited the La Selva Basin, and it is classified as phreatomagmatic eruption, the most common in this area (Martí et al. 2011).  The earliest geologic study considered the Camp dels Ninots sediments as lacustrine deposits without any mention to volcanism (Vidal 1882), being Vehí et al. (1999) who identified these sediments as the infill of a maar. These maar sediments have been extensively studied due to their well-preserved Pliocene and Pleistocene fossil record (Campeny & Gómez de Soler, 2010; Gómez de Soler et al., 2012; Jiménez-Moreno et al., 2013; Claude et al. 2014). More recently, a detailed geological study of maar structure and infill sediments based on 8 cores and 9 electric resistivity tomography cross sections was reported by Oms et al. (2015).  The base of the CNMD is made of Late Carboniferous to Permian granites, schists, and Pliocene arkose clays, sands, and gravels from the La Selva Basin alluvial system (Figure 1). The lacustrine sediments fill a ring volcanic structure made of Pliocene tephra and have been classified in three main sedimentary facies: (i) white colour carbonates; (ii) green colour clays without organic matter, and (iii) black colour clays rich in organic matter. The CNMD lacustrine sediments are characterized by frequent early diagenetic silicified horizons, which appear as chert nodules, and dolomitized horizons. Both types of diagenetic products, chert nodules and dolomite, are the object of this study  METHODS  This study is based in two boreholes drilled in 2015 time (see also Bolós et al., this volume): Can Pla 1 (CP1) and Can Pla 2 (CP2) (Table 1 & Figure 1c).  Well logging of CP1 and CP2 was performed by personnel of the Borehole geophysical logging laboratory at Institute of Earth Science Jaume Almera – CSIC (CSIC, Barcelona, Spain). A complete set of geophysical logs was acquired (table 1), including oriented televiewer images. The images were processed at the ICTJA-CSIC lab by specialists using WellCAD (ALT) and Techlog (Schlumberger) log processing softwares. The CP1 well core sampled in order to elaborate thin sections (27) and collect powder samples (27) for X-Ray Diffraction (XRPD). XRPD scans of all samples hand-ground powder samples were acquired using a Bruker D8-A25 powder diffractometer equipped with a Cu x-ray source (Cu Kα radiation) and a LynxEye TABLE 1: Characteristics of the CP1 and CP2 wells drilled in the Camp dels Ninots. IX Congreso Geológico de España                                                                                                                   Geo-Temas, 16 (2) ISSN 1576-5172 577  FIGURE 2: Log responses of the CP1 well & CP2 well. A) Section of the CP1 well between 42.5 m and 45 m. Televiewer response of the core (left column). The right column is the core picture of the same section. . It can be seen that the carbonate massive layers produce a high reflectance in a log response; B) Televiewer response from 55 m to 58 m of the CP2 well with different facies: nodular in the upper part and plane parallel into the middle and to the bottom; C) Another section of CP1 well between 55.25 m and 55.75 m. Televiewer response and the core picture can be observed on the left and right images, respectively. The televiewer response of the silica nodules can be observed in the picture; D) Resistivity and sonic log responses of the same section as in C (nodular facies).   position sensitive detector. The scans were recorded between 3º and 65º (in 2θ) with a 0.05° step size and equivalent integration times of 576 s per step. Phase identification was performed by using the Diffrac.EVA software in combination with the Powder Diffraction File (PDF-2) and the Crystallography Open Database (COD).  RESULTS AND DISCUSSION  Televiewer image of CP1 and CP2 wells shows two types of stretches with high reflectance response. The first one is a continuous signal that is parallel to bedding and can be identified throughout the width of the well. The second one is irregular and discontinuous (Fig. 2A, 2B & 2C).  CP1 well core examination allows identifying the plane parallel continuous stretches as indurate carbonate horizons located between soft sediments (Fig. 2A). The XRPD analyses of these carbonate horizons reveals that are made of dolomite with minor amounts of other carbonates (ankerite & calcite) and feldspars (plagioclases & potassium feldspar). In a similar way, CP1 well core examination of the irregular and discontinuous stretches allows identifying these areas as chert nodules located within carbonates and clays (Fig. 2C). The XRPD analyses of the nodules reveals that are made of opal-A and opal-CT while the horst rock are composed of carbonate (dolomite & ankerite & calcite) clays (smectite & illite & kaolinite), feldspars (plagioclases & potassium feldspar) and quartz.  The excellent correlation between televiewer images and mineralogical composition of the reported high reflectance stretches in the CP1 well can be used to infer mineralogical composition of the CP2 televiewer imaging along sections not cored and/or with poor core recovery. Together with televiewer, other geophysical signal such as resistivity or sonic logs also register a characteristic response of levels with dolomite or chert nodules horizons (Figure 2D).  CONCLUSIONS  The combination of geophysical logs, televiewer oriented images logging and direct core examination (mineralogical determinations) has allowed us to characterize horizons rich in opal nodules and levels with dolomite.  The televiewer image appearance of opal nodules is that of discontinuous reflective shapes while the carbonate layers appear as continuous highly reflective continuous bands.  Our results obtained by comparing geophysical logs and cores indicate that ultrasonic high resolution images obtained with the acoustic televiewer  are a good tools to identify even relativelysmall sized (3 – 5cm) opal nodules.  IX Congreso Geológico de España                                                                                                                   Geo-Temas, 16 (2) ISSN 1576-5172 578 ACKNOWLEDGEMENTS  Drilling was financed by El Plio-pleistocè del Camp dels Ninots i la depressió prelitoral: evolució paleoclimàtica, dispersions faunístiques i humanes (2014-100575) project (Departament de Cultura de la Generalitat de Catalunya) and SGR2014-901 (AGAUR, Generalitat de Catalunya). This research was funded by Repsol through a collaborative project with the University of Barcelona. The authors would like to thank the Town Council of Caldes de Malavella for providing logistics and the land owners (Mr. Jaume Pla and family). ). Techlog (Schlumberger) software was used to process and interpret ultrasonic borehole images  REFERENCES  Campeny, G. & Gómez de Soler, B. (2010): El Camp dels Ninots: rastres de l’evolució (Eds.). Edited by Ajuntament de Caldes de Malavella—IPHES. 199 pp.  Claude, J., Gomez de Soler, B., Campeny, G., Agustí, J. & Oms, O. (2014): Presence of a chelydrid turtle in the late Pliocene Camp dels Ninots locality (Spain). Bull Soc Geol France 185(4), 253–256 Gómez de Soler, B., Campeny Vall-Llosera, G., van der Made, J., Oms, O., Agustí, J., Sala, R., Blain, H.A., Burjachs, F., Claude, J., García Catalán, S., Riba, D., Rosillo, R. (2012): The Camp dels Ninots (NE Spain): a new key locality for the Middle Pliocene terrestrial faunas of Europe. Geologica Acta 10 (2), 1–17. Jiménez-Moreno, G., Burjachs, F., Expósito, I., Oms, O., Carrancho, Á., Villalaín, J.J., Agustí, J., Campeny, G., Gómez de Soler, B. & van der Made, J. (2013): Late Pliocene vegetation and orbital-scale climate changes from the western Mediterranean area. Global Planet Change 108, 15–28. Martí, J., Planagumà, L., Geyer, A., Canal, E., & Pedrazzi, D. (2011): Complex interaction between Strombolian and phreatomagmatic eruptions in the Quaternary monogenetic volcanism of the Catalan Volcanic Zone (NE of Spain). Journal of Volcanology and Geothermal Research, 201(1), 178-193. Oms, O., Bolós, X., Barde-Cabusson, S., Martí, J., Casas, A., Lovera, R.,  Himi, M., Gómez de soler, B., Campeny Vall-Llosera, G., Pedrazzi, d. & Agustí, J. (2015): Structure of the Pliocene Camp dels Ninots maar-diatreme (Catalan Volcanic Zone, NE Spain). Bulletin of Volcanology, 77(11), 1-13. Pous, J., Solé Sugrañes, L., & Badiella, P. (1990): Estudio geoeléctrico de la depresión de La Selva (Girona). Acta geológica hispánica, 25(4), 261-269. Vehí, M., Pujadas, A., Roqué, C. & Pallí, L. (1999): Un edifice volcanic inèdit a Caldes de Malavella (La Selva, Girona): el Volcà del Camp dels Ninots. Quaderns de la Selva, 11. Centre d’Estudis Selvatans. Vidal, L. M. (1882): Estudio geológico de la estación termal de Caldes de Malavella. Bol. de la Comisión del Mapa Geol. de España, nº 9, p. 65-91. Wright, V. P. (2012): Lacustrine carbonates in rift settings: the interaction of volcanic and microbial processes on carbonate deposition. Geological Society, London, Special Publications, 370(1), 39-47. 

Borehole image techniques applied to identification of chert and dolomite layers in lacustrine sediments

Borehole image techniques applied to identification of chert and dolomite layers in lacustrine sediments

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